A ten-fold increase in some types of bacteria living under the foreskin can increase a man’s risk of HIV infection by up to 63 percent, according to a new study at the George Washington University. This study, which was published in the journal mBio, shows for the first time that penile bacteria may be a previously unrecognized risk factor for HIV infection in men.

By examining the collection of bacteria living under the foreskin of men who either went on to become infected by HIV or remained negative, the researchers showed a connection between the amount of anaerobes, bacteria that thrive in low-oxygen environments and an increased risk of becoming infected by HIV.

They conducted this study using penile swabs collected from uncircumcised heterosexual men in Rakai, Uganda during a large, two-year clinical study.